1. Field of the Invention
The present invention relates to a personal computer with a flat panel display and more specifically, to a battery operable personal computer.
2. Description of the Related Art
Various kinds of so-called laptop personal computers are recently being developed as portable personal computers. A specific laptop computer has a flat panel display, such as a liquid crystal display. The liquid crystal display is hinged to its computer body to swing from a closed position to an open position or vise versa. The liquid crystal display covers a keyboard which is built in its computer body when it is closed. Accordingly the computer is more easily carried around. The flat panel display, such as the liquid crystal display, is preferable for a laptop personal computer to improve the portability of the computer.
The laptop personal computer includes a battery so that it can be operated at any places, even where the commercially-available power supply is not provided. When the battery is almost discharged, an operator removes the battery from the computer body, and can charge the battery again, or replace it with a new one. While charging or replacing the battery, the operator has to use the commercially-available power supply to operate the laptop computer.
Recently, various means have been developed to prolong the service life of a battery. One of these means is to control a display luminance to reduce the power consumption of a display.
Generally, energy is converted in the form of luminance to provide a data display. The power consumption varies according to the luminance level.
For example, the display luminance rises as the electric power of a light source increases in a liquid crystal display using, as a light source, a back light that illuminates the display from the back by a plane luminophor, such as an electroluminescence (EL) panel, or a side light that illuminates the display from the side by a cold-cathode tube (fluorescent (FL) tube). In a plasma display, as electric discharge in the panel, i.e., power consumption by discharge, increases, the display luminance rises.
As described above, the power consumption varies in accordance with the luminance level of the display. Conventionally, therefore, the luminance is automatically varied between when the computer is driven by the battery and when it is driven by the commercially-available power supply; the luminance has a lower value during the battery-operated period than during the period in which the commercially-available power supply is used. The power consumption in the battery-operated period can be reduced in this manner, thereby prolonging the service life of the battery.
The luminance is, however, fixed to a given low level during the battery-operated period. The display luminance may appear too low for some operators that it is difficult for the operators to see the display screen. Or, some other operators may wish to set the display luminance lower to prolong the life of the battery.